1. Field of the Invention
The present invention relates to a method and apparatus that forms a ball at a tip end of bonding wire via electrical discharge.
2. Prior Art
A ball formation apparatus used in a wire bonding machine makes a ball at the end of bonding wire. In such an apparatus, high voltage is applied and discharged between the end of a bonding wire and a discharge electrode. Examples of these kind of apparatuses are shown in the Japanese Patent Application Publication Nos. 61-46968, 1-54858, and 1-54859.
Generally, in the ball forming process, if the distance between the wire end and the discharge electrode differs for each gap, and high voltage from a high-voltage power source is applied to the gap via a constant-current circuit, such inconsistent gaps cause ball formations in different diameters. In other words, since it is necessary to raise the discharge voltage as the gap becomes larger, the diameter of the ball tends to become larger than desired.
In the above-described prior art, no consideration whatsoever is given to how the differences in the gap between the discharge electrodes and the wire ends are compensated. As a result, uniform size balls are not obtainable even though there is a steady application of high-voltage to the gaps.